The manipulation and transfer of tiny liquid droplets is an operation that is typically done for the purpose of maneuvering targeted tiny droplets (capture and release) or preparing tiny liquid droplets from a bulk source. Such a manipulation operation is a significant and pragmatic technique that features prominently in both research and industry. Precise and reliable manipulation of droplets is a critical step for chemical and biological reactions and analysis processes, such as microfluidics and micro-reactors. The quality of reacted products and the accuracy of analysis highly depend on precise volume control of manipulation and transfer processes. Manipulation with noticeable liquid loss can lead to unsatisfactory reaction products and erroneous analytical results.
With help of measurement tools, preparing and transferring large amounts of liquid with specific volumes is usually easy. However, for tiny volume liquid droplets, such as those of microliter and nanoliter size, the same processes are difficult due to the relatively considerable surface tension encountered. In particular, in order to capture and manipulate tiny liquid droplets reliably, it is essential that the manipulator provide high liquid/solid adhesion so as to overcome the substantial surface tension of the droplet and to balance the gravitational force acting on the droplet. On the other hand, in order to disperse or release the droplets on a target surface, the liquid/solid adhesion of the manipulator should be greatly reduced so that the droplet will be released due to the prevailing gravitational force. These two seemingly contradictory requirements lead to challenges for tiny droplet manipulation. Moreover, unlike conventional measurement methods with large amounts of liquid residue remain on the measurement tools after the manipulation, in order to control tiny droplet volume precisely, the transfer process for these tiny droplets should be nearly loss-free.
To prepare droplets with volumes down to the nanoliter size, various techniques, including pyroelectrodynamic shooting, piezoelectric nozzle dispersing, and focused acoustics ejection, can be applied. However, none of these techniques can deposit droplets on a liquid-repellent surface reliably due to the low liquid/solid adhesion of such a surface. Although these techniques can aliquot bulk liquid source into small droplets, they cannot manipulate individual droplets on-demand. Such weak maneuverability over droplets makes them inapplicable for applications requiring multistage manipulation, such as micro-reactors and multi-component particle synthesis. Furthermore, these techniques often require expensive components (such as an infrared laser or a focused acoustic transducer) and complicated fabrication processes. See, for example Ferraro, P., Coppola, S., Grilli, S., Paturzo, M., & Vespini, V., “Dispensing nano-pico droplets and liquid patterning by pyroelectrodynamic shooting,” Nature nanotechnology, 5(6), 429-435 (2010); and Ellson, R., Mutz, M., Browning, B., Lee, L., Miller, M. F., & Papen, R., “Transfer of low nanoliter volumes between microplates using focused acoustics—automation considerations,” Journal of the Association for Laboratory Automation, 8(5), 29-34 (2003), both of which are incorporated herein by reference in their entirety.
Smart non-wettable surfaces with responsive liquid adhesion stimulated by various external stimuli, such as pH value and temperature, have been proposed for liquid handling and transfer. However, most of such techniques still suffered from long responding time. More importantly, due to conventional irreversibility from Wenzel to Cassie state, most of their switching adhesion are ex-situ, which means a different liquid droplet is required to study the adhesion change after the switch (if the surface changes from adhesive to nonsticky, the droplet deposited before the switch will still be pinned, whereas a newly deposited droplet can roll off easily). Therefore, such smart surfaces are still not pragmatic for real-time droplet transfer. See for example, Cheng, Z., Lai, H., Du, M., Zhu, S., Zhang, N., & Sun, K., “Super-hydrophobic surface with switchable adhesion responsive to both temperature and pH,” Soft Matter, 8(37), 9635-9641(2012), which is incorporated herein by reference in its entirety.
Several methods have been proposed to enable in-situ switchable solid/liquid adhesion for on-demand droplet capturing and releasing. For example, an adhesive superhydrophobic surface for superparamagnetic microdroplets, superhydrophobic “aspirators,” and curvature-driven switching surfaces. However, these methods all have their drawbacks. Adhesive superhydrophobic surfaces for superparamagnetic microdroplets are only applicable to droplets containing magnetic nanomaterials. The inclusion of magnetic nanomaterials may impede analysis and may even be incompatible with chemical and biological components in the droplets. For superhydrophobic “aspirators,” small droplets evaporate quickly due to generated negative pressure and their portability is impaired by an externally attached vacuum pump. Switching processes for curvature-driven switching surfaces are difficult to perform because a curvature has to be induced by deforming a surface from the backside. Moreover, none of these methods have proved to be able to manipulate oil droplets. See for example, Hong, X., Gao, X., & Jiang, L., “Application of superhydrophobic surface with high adhesive force in no lost transport of superparamagnetic microdroplet,” Journal of the American Chemical Society, 129(6), 1478-1479 (2007); Guo, D., Xiao, J., Chen, J., Liu, Y., Yu, C., Cao, M., & Jiang, L. Superhydrophobic “Aspirator”: Toward Dispersion and Manipulation of Micro/Nanoliter Droplets,” Small, 11(35), 4491-4496 (2015); and Wu, D., Wu, S. Z., Chen, Q. D., Zhang, Y. L., Yao, J., Yao, X., & Sun, H. B., “Curvature-Driven Reversible In Situ Switching Between Pinned and Roll-Down Superhydrophobic States for Water Droplet Transportation,” Advanced Materials, 23(4), 545-549 (2011), which are incorporated herein by reference in their entirety.
It would be advantageous to have a method that could prepare and manipulate tiny volumes of droplets in real-time, without the drawbacks of the prior art.